Radio communication systems are becoming more reliable, and the Mean Time Between Failure (MTBF) associated therewith is high. However, in microwave radio transmission, the associated transmission link may be long and multipath may be frequently encountered. Multipath refers to multiple transmission paths between transmit and receive antennas of a communication system. Multipath may result in both frequency-selective fading and space-selective fading. Frequency-selective fading generally indicates that a channel varies with frequency. Space-selective fading generally indicates that a channel is dependent upon the position of the respective transmit and receive antennas. When multipath interferes with reception of a radio transmission signal, the received signal is distorted causing errors in the corresponding demodulated data stream.
FIG. 1A illustrates an arrangement of symbol points in accordance with a 4 QAM (Quadrature Amplitude Modulation) method on an I-Q coordinate plane. With reference to FIG. 1A, a symbol point of a received signal corresponds to any of four signal points positioned concentrically on the I-Q coordinate plane. Therefore, it is possible to transmit at one time 2 bits of data representing any of the four signal points.
Advances in radio communication systems, however, require data transmission of larger volume at higher speeds. Accordingly, multiple value (M-ary) modulation methods having values larger than the 4 QAM modulation method described above have been developed. As an example of such an M-ary modulation method, 16 QAM is commonly employed in data communications. FIG. 1B is an illustration of an arrangement of symbol points in accordance with the 16 QAM modulation method on the I-Q coordinate plane. With reference to FIG. 1B, a symbol point of a received signal corresponds to any of a total of 16 signal points on the coordinate plane, arranged four by four in a lattice form in each quadrant of the I-Q coordinate plane. Therefore, it is possible to transmit at one time 4 bits of data representing any of the 16 signal points.
When a modulation method having a larger M-ary number is employed and the communication environment of the propagation path is defective (i.e., if the propagation path has severe interference, noise or encounters multi-path), symbol points may be recognized erroneously since the interval between each of the symbol points is narrow and the symbol points are arranged tightly in the respective modulation method, as may be seen from the arrangement of symbol points of FIG. 1B. Therefore, though this method has a communication speed faster than the 4 QAM modulation method illustrated in FIG. 1A, it is more susceptible to reception errors.
In a radio communication environment prone to multipath, several known techniques may be implemented to mitigate the effects of multipath. Prior art techniques commonly used to protect a signal path include switching from an online channel, receiver or antenna to a standby channel, receiver or antenna by 1+1 Frequency Diversity (FD) or 1+1 Space Diversity (SD) or combination thereof.
SD may commonly be provided by utilizing multiple receive antennas separated by a sufficient distance to take advantage of space-selective fading. With reference to FIG. 2, a prior art method is illustrated that utilizes SD with two receive antennas including a separate receiver connected to each receive antenna. A pair of antennas 201, 202 are coupled to respective receivers 203, 205 that demodulate the signals received at each antenna. An antenna selection circuit 209 accepts the demodulated output of the receivers and provides control to an antenna switch 207 to select a data set having the least amount of error.
The aforementioned diversity techniques and examples, however, are not always appropriate for evaluating the communication quality of the propagation path. For example, different radio reception apparatuses employ different methods of reception and performances. Qualities of components, such as filters used in the reception apparatuses, vary and such differences and variations have an influence on the quality of communication. Conventional parameters such as reception level, frame error rate, and carrier to interference ratios (CIR) do not reflect such quality or performances of the reception apparatuses. Further, as may be seen from a comparison of the modulation methods shown in FIGS. 1A and 1B, even when there is no reception error with a modulation method having smaller M-ary values, it is unpredictable whether there arises reception error or not with another modulation method (16, 64, 128, 256 QAM) having a larger M-ary values (i.e., having dense symbol points on the I-Q plane). As a result, special and complicated procedures and hardware are necessary to measure the respective conventional parameters of the propagation path during communication.
Thus, there is a need in the art for a system and method of selecting antennas or receivers in a multipath environment without incurring any errors in the received signal.
Accordingly, it is an object of the present subject matter to obviate many of the deficiencies in the prior art and to provide a novel method of switching from a first receiver receiving a constant bit rate signal to a second receiver receiving the constant bit rate signal, where the constant bit rate signal received by the first and second receivers is converted to a first baseband signal and a second baseband signal. The method further comprises the steps of estimating a signal quality metric of the first baseband signal, comparing the signal quality metric to a predetermined threshold, and switching from the first baseband signal to the second baseband signal if the signal quality metric is greater than the threshold.
It is also an object of the present subject matter to provide a novel method of switching from a first receiver receiving a constant bit rate signal to a second receiver receiving the constant bit rate signal, where the constant bit rate signal received by the first and second receivers is converted to a first baseband signal and a second baseband signal, respectively. The method further comprises the steps of estimating a first signal quality metric of the first baseband signal, estimating a second signal quality metric of the second baseband signal, comparing the first signal quality metric to the second signal quality metric, and switching from the first baseband signal to the second baseband signal if the first signal quality metric is greater than the second signal quality metric.
It is another object of the present subject matter to provide a novel system for switching from a first receiver receiving a constant bit rate signal to a second receiver receiving the constant bit rate signal. The system comprises a first converting circuit for converting the signal from the first receiver to a first baseband signal and a second converting circuit for converting the signal from the second receiver to a second baseband signal. The system further comprises an estimating circuit for estimating a first signal quality metric of the first baseband signal, a comparing circuit for comparing the first signal quality metric to a predetermined threshold, and a switching circuit for switching from the first baseband signal to the second baseband signal if the first signal quality metric is greater than the predetermined threshold.
It is still an object of the present subject matter to provide a novel system of switching from a first receiver receiving a constant bit rate signal to a second receiver receiving the constant bit rate signal comprising a first converting circuit for converting the signal from the first receiver to a first baseband signal and a second converting circuit for converting the signal from the second receiver to a second baseband signal. The system also comprises a first estimating circuit for estimating a first signal quality metric of the first baseband signal, a second estimating circuit for estimating a second signal quality metric of the second baseband signal, a comparing circuit for comparing the first signal quality metric to the second signal quality metric, and a switching circuit for switching from the first baseband signal to the second baseband signal if the first signal quality metric is greater than the second signal quality metric.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.